In a sentence where we have two listed words that are hyphenated, we can omit the latter part of the first compound and still be grammatically correct:
I don't believe we will ever find helium-based or hydrogen-based life forms.
I don't believe we will ever find helium- or hydrogen-based life forms.
However, if we have two (related) words which both end in the same suffix, can we still apply this notation? I've seen this used before, but I'm not sure it's proper:
It doesn't matter whether the character is a protagonist or antagonist.
It doesn't matter whether the character is a pro- or antagonist.
More often than in writing, I hear this in spoken conversation, usually with an emphasis on the prefixes (i.e. "... a pro- or antagonist ..."), as though there is actually a hyphen in both words.
I've read up a small bit on conjugation reduction here (thanks to search actually finding that term for me), but it doesn't seem to answer this particular question.
Is the reduction of non-hyphenated words allowed within English grammatical rules?